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How do seahorses differ from all other animals?

Seahorses and their close relatives, sea dragons, are the only species in which the male gets pregnant and gives birth.

a seahorse; image courtesy of Sunshine City Aquarium, Tokyo, Japan

Male seahorses and sea dragons get pregnant and bear young—a unique adaptation in the animal kingdom.

Seahorses are members of the pipefish family. In addition to their iconic appearance, seahorses possess many interesting attributes. Among them are specialized structures in their skin cells, called chromatophores, which allow the mostly sessile seahorses to change color to mimic their surroundings. Well camouflaged as they cling to stalks of seagrass in their shallow habitats, seahorses can be hard to see.

Their truly remarkable biological claim to fame, however, is that male seahorses and sea dragons get pregnant and bear young—a unique adaptation in the animal kingdom.

After completing an elaborate courtship dance that may go on for hours or days, the female seahorse transfers her mature eggs into the male’s brood pouch, where they are fertilized. At the end of a gestation period usually lasting from two to four weeks, the pregnant male’s abdominal area begins to undulate rhythmically, and strong muscular contractions eject from a few dozen to as many as 1,000 fully formed baby seahorses into the surrounding water. After that, the offspring must fend for themselves. Large litters are necessary because only about 0.5 percent will survive to adulthood.

Many, if not all, of the 47 known seahorse species—14 of which were identified only in the 21st century—are in decline worldwide.

Because seahorses generally live in shallow, near-coastal waters, human activities including development, pollution, fisheries, and traditional medicine have reduced their numbers. At the same time, their universal appeal has worked against them; until recently, wild seahorses were often captured for the aquarium trade. The delicate creatures tend to fare poorly in aquaria, however. In recent years, captive-bred seahorses have shown promise as hardier tank-dwellers than their wild relatives.